We plan to further the study of the function of SIRT1 in mice. In Aim 1 we study whether SIRT1 (and other sirtuin) activity declines aging. This study will include a detailed analysis of NAD and NADH levels in cellular compartments of muscle, heart and brain. In Aim 2 we will determine the effects of depletion or augmentation of SIRT1 in brown fat, muscle, heart and 2-cells. This aim will employ tissue specific deletion and over- expression of SIRT1 in mice. In Aim 3 we will study the effects of SIRT1 on bone. This study will use mice knocked out for SIRT1 in osteoblasts or osteoclasts to determine mechanisms by which SIRT1 determines bone density. This aim will also study how calorie restriction affects bone. In Aim 4 we will study the role of SIRT1 in T cells. In this aim we will study immune function in T cell-specific SIRT1 knockout mice and determine the mechanism by which SIRT1 functions in these immune cells.